The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus for forming an image on an object to be illuminated by causing a light beam modulated in correspondence with an image to be formed to scan the object to be illuminated.
Conventionally, an image forming apparatus is known in which a light beam such as a laser beam emitted from an optical scanning device is caused to scan an image carrier (horizontal scanning) to form an electrostatic latent image on the image carrier, and a toner image obtained by developing the electrostatic latent image is transferred onto a recording material, thereby forming an image on the recording material. Also known is a color image forming apparatus which has a plurality of optical scanning devices and image carriers, electrostatic latent images are formed independently on the image carriers, respectively, to form toner images in respective colors (e.g., C, M, Y, and K), and the toner images in the respective colors are consecutively transferred onto and superposed on an identical recording material, thereby forming a color image on the recording material.
In this type of image forming apparatus, there are cases where the positions where toner images in the respective colors are formed on the image carriers and the positions where the toner images in the respective colors are transferred onto the recording material are misregistered in the horizontal scanning direction with respect to predetermined positions due to such factors as (1) tolerances of optical components which make up the optical scanning device and tolerances of mounting positions of the optical components, (2) tolerances of a frame on which the optical scanning device is mounted, and (3) changes in the surrounding environment such as the temperature and the state of installation or changes with the passage of time. This misregistration in the transfer position is visually recognized as a color misregistration in the formation of a color image, and resulted in an appreciable decline in the image quality.
It should be noted that, as shown in FIG. 10 in greater detail, the positional misregistration and color misregistration along the horizontal scanning direction are made up of three elements including (1) a displacement in the recording magnification of an overall image (a change in the length of an image region along the horizontal scanning direction), (2) a partial variation in the recording magnification of the image (a partial change in the length of the image region along the horizontal scanning direction; by way of example, the drawing shows a case in which the length of a partial region on the scan starting side with the center of an image region serving as a boundary is longer than a partial region on the scan terminating side), and (3) an misregistration in the writing position (a positional misregistration along the horizontal scanning direction in the image region).
In connection with the above description, it is described in the Unexamined Japanese Patent Application Publication No. Hei 2-291573 that, in order to improve registration in the horizontal scanning direction of a tandem engine, the displacement in the magnification is corrected by detecting the position of a registration mark of each color by a sensor and by changing a pixcel clock frequency for each color by means of a phase-locked loop (PLL), the displacement in the partial magnification is corrected by rotating or moving parallel an f.theta. lens by a driving means such as a piezoelectric element or the like, and the misregistration in the writing position is corrected by changing the image writing timing. However, if the f.theta. lens is rotated or moved parallel, there is a possibility that the focusing position of the laser beam deviates from a photoconductor and the sharpness of the image declines, and there is a problem, among others, that the cost of the driving means rises since it is necessary to precisely control the amount of movement of the f.theta. lens by its disposition.
In addition, it is described in the Unexamined Japanese Patent Application Publication No. Hei 6-242386 that, in order to correct the positional misregistration of dots due to an f.theta. error, a pixel clock is generated by separating a pixel clock frequency outputted from a source generator, and the separating ratio is changed by a programmable counter, thereby controlling the period of the pixcel clock for each dot. It should be noted that in the above-described publication, the separating ratio data is stored in a memory for each dot.
However, the above-described technology is difficult to be applied to a case such as where an image of a high resolution is formed at high speed. Namely, under the conditions where the recording density of the image is 600 spi (spi denotes the number of light spots per inch) and the process speed (the moving speed of the photoconductor) is approximately 260 mm/s, even if an attempt is made to expose and record two lines simultaneously by a dual LD, the pixel clock frequency needs to be set to a high frequency of 300 MHz or more. In addition, if an attempt is made to correct the color misregistration or positional misregistration with a high resolution, the separating ratio data needs to be stored for each step of the resolution, so that there is a problem in that memory of a huge memory capacity is required.